Firefox 4 thumps IE9 in first day download contest

Unlike Microsoft's IE9, Firefox runs on Windows XP, still the world's most popular OS

Earlier this week, Mozilla knocked its rival for leaving XP users stuck with the two-year-old IE8, and with no hope for a Microsoft-made browser upgrade.

Firefox could use the jumpstart provided by version 4: Net Applications' statistics show that the browser, once a lock to grab a quarter of the worldwide browser usage share, has stalled since its November 2009 peak. In the last 12 months, Firefox has lost two-and-a-half percentage points of share, and now accounts for 21.7% of all browsers in use.

During that same 12-month period, IE has lost 4.8 percentage points, while Google's Chrome gained 5.3 points and Apple's Safari grew by 1.9 points.

Users can download Firefox 4 for Windows, Mac OS X and Linux from Mozilla's site. People already running Firefox 3.5 or 3.6 can grab the upgrade by selecting "Check for updates" from the browser's Help menu.